There have actively been proposed apparatuses and methods which change and control damping forces of shock absorbers disposed between the vehicle body and wheels. For example, Japanese Patent Application Laid-Open (kokai) No. 2007-8373 (Patent Document 1) discloses a suspension-characteristic computation method which provides a design index of a suspension in consideration of the correlation between roll and pitch generated in the vehicle body. In this suspension-characteristic computation method, a pitch moment determined by the geometries of suspensions is computed as the sum of a front-wheel-side ascending/descending force and a rear-wheel-side ascending/descending force. The front-wheel-side ascending/descending force is represented by the product of a front-wheel-side geometry proportional coefficient and the square of a tire lateral force. The rear-wheel-side ascending/descending force is represented by the product of a rear-wheel-side geometry proportional coefficient and the square of a tire lateral force. Further, a pitch moment determined by damping forces of the suspensions is computed from the product of a damping force proportional coefficient and a roll rate. A pitch angle is then computed from the sum of the two calculated pitch moments and the product of the gain and phase delay of the pitch angle in relation to the pitch moment, and a phase difference between the pitch angle and the roll angle is computed on the basis of this computed pitch angle.
In the case where suspensions are designed in accordance with such a suspension-characteristic computation method, the timings of generations of a roll and a pitch can be synchronized through proper setting of an expansion difference and a contraction difference between shock absorbers disposed on the front wheel side and shock absorbers disposed on the rear wheel side. As a result, maneuvering stability can be improved.
Further, Japanese Patent Application Laid-Open (kokai) No. H06-99714 (Patent Document 2) discloses a vehicle suspension apparatus which can perform active roll suppression control in accordance with the roll direction of the vehicle body by use of only a steering sensor. In this vehicle suspension apparatus, when a steering angle detected by the steering sensor exceeds a predetermined neutral threshold, control is switched into a roll control mode for controlling left and right shock absorbers to have large damping forces during expansion or contraction thereof, on the basis of the roll direction of the vehicle body determined from the polarity of a steering angular speed. For a reverse steering performed thereafter, the apparatus controls the damping forces of the left and right shock absorbers such that their damping forces change in a direction opposite the direction in which the damping forces are changed in the above-described roll control mode, when the polarity of the steering angular velocity reverses.
Further, Japanese Patent Application Laid-Open (kokai) No. H06-48147 (Patent Document 3) discloses a vehicle suspension apparatus which suppresses roll stemming from abrupt steering, and prevents riding quality from deteriorating when a steering operation is performed. In this vehicle suspension apparatus, a control signal is calculated from a bounce rate based on sprung-portion ascending/descending speed, a pitch rate detected from a difference of sprung-portion ascending/descending speed between the front and rear sides of the vehicle body, and a roll rate detected from a difference of sprung-portion ascending/descending speed between the left and right sides of the vehicle body. When the control signal is equal to or greater than a predetermined large threshold, the damping forces of shock absorbers on the expansion side (the side corresponding to the steering direction) are increased, and the damping forces of shock absorbers on the contraction side (the side opposite the side corresponding to the steering direction) are decreased. Further, when the control signal is equal to or less than a predetermined small threshold, the damping forces of shock absorbers on the expansion side are decreased, and the damping forces of shock absorbers on the contraction side are increased.